It has long been known in the art that images may be prepared using what is known as a "wash-off" development step in which a latent image is developed by washing with water or other aqueous or non-aqueous developing solution. Such techniques have been widely used in the preparation of flexographic plates, printed circuits, etc. from photosensitive compositions.
Diffusion patterning techniques have also been developed, primarily in the electronics arts, for the preparation of conductors, dielectrics and resistors from photosensitive or non-photosensitive thick films. For example, U.S. Ser. No. 07/768,504, filed Sep. 30, 1991, teaches a method and composition for diffusion patterning in which an unpatterned layer comprising a solid dispersion of a solid organic acid and a plasticizer is patterned with a second layer comprising an organic base and a volatile solvent. After the patterning layer has been applied, such as by screen printing, thermal transfer or ink jet printing, the element is heated to evaporate the solvent and diffuse the organic base into the underlying unpatterned layer. The reaction of the organic base with the acidic polymer renders the latter soluble in the patterned areas. The element is then developed by washing with an aqueous solution having a pH of 5 to 8.5.
Similarly, U.S. Ser. No. 07/790,863, filed Nov. 14, 1991, teaches a method and composition for diffusion patterning in which the unpatterned first layer comprises an acid labile polymer composition and the patterning layer comprises an organic acid and a volatile solvent. After heat is applied to evaporate the solvent and diffuse the organic acid, the acid labile polymer in the patterned areas is susceptible to being washed-off with an aqueous solution having a pH of at least 7.
It is also known in the art that relief images may be prepared from photosensitive or non-photosensitive elements using a wash-off development techniques in which the image or patterned layer is applied with an ink jet printer. British Patent 1 431 462 teaches such a method. More specifically, this reference teaches both positive and negative-working systems for forming a relief image using primarily natural proteinaceous materials. In the negative-working system, a polymer coating (e.g. gelatin) is crosslinked, polymerized, or otherwise imagewise hardened by applying a suitable agent (e.g formaldehyde) using ink jet printing and then dissolving the unhardened areas with a developing solution. In the positive-working system ink jet printing is used to apply an agent (e.g enzyme) which will degrade an otherwise insoluble polymer coatings thus rendering it soluble in the developing solution.
Although desirable in principle, the methods disclosed in the aforementioned British Patent are disadvantageous in practice. First, the natural proteinaceous polymers disclosed in this reference lack many physical characteristics necessary from most relief applications, thus rendering such reliefs of limited practical or commercial utility. Second, the crosslinking and degredation reactions are relatively slow, particularly at room temperature, thus decreasing the efficiency of the process. The patent teaches that heat could be applied to facilitate the reaction, but this requires an additional step and presents other complications. Third, many of the crosslinking agents, such as aldehydes, are environmentally disadvantageous Fourth, the use of photopolymerizable compositions requires an additional exposure step, which increases costs and lowers efficiency. Fifth, many of the developing solutions are organic solvent based, are highly alkaline, or are maintained at elevated temperatures and thus present additional environmental and safety hazards. Sixth, the latent images generated by such methods are not stable, reducing the utility of the process. Finally, the resolution obtainable from these methods was only marginal and often insufficient for demanding commercial applications.
The diffusion patterning methods discussed above require a separate step to evaporate the solvent and diffuse the patterning layer into the unpatterned layer.
The present invention overcomes many of these disadvantages and provides a process which poses minimal environmental risks, is easy to use and has the capability of producing images of high resolution in a very short period of time.